[testsuite] [riscv] limit mcpu-xiangshan-nanhu.c to rv64

[testsuite] [riscv] limit mcpu-xiangshan-nanhu.c to rv64

The testcase makes the -march option conditional on rv64, and #errors
out if the desired CPU properties are not active.  This makes the test
fail on rv32.  Arrange to skip the test on rv32 instead, moving the
rv64 conditional.

for  gcc/testsuite/ChangeLog

	* gcc.target/riscv/mcpu-xiangshan-nanhu.c: Skip on non-rv64.